I've been experimenting as well. Seems to only work in the skins_dds subfolder....In versions 0.5X onwards. I was hoping to implement the technique on the *_interior1.dds file so you could see the sunroof from the inside as well, but because you MUST add the _____ALPHA.dds to the name in order for the effect to show, it is not possible
i.e. cannot change the names in the DDS subfolder, where as in the skins_dds folder, you can name your skin whatever you want.